How to use the assemblyscript/lib/loader.instantiate function in assemblyscript

this.onmessage = function (evt) {

  // message from UI thread
  var msg = evt.data 
  switch (msg.purpose) {

    // Message: Load new wasm module

    case 'wasmModule': 
      // Instantiate the compiled module we were passed.
      wasm = loader.instantiate(msg.wasmModule, importObj)  // Throws
      // Tell nBodySimulation.js we are ready
      this.postMessage({ purpose: 'wasmReady' })
      return 


    // Message: Given array of floats describing a system of bodies (x,y,x,mass), 
    // calculate the Grav forces to be applied to each body

    case 'nBodyForces':
      if (!wasm) throw new Error('wasm not initialized')

      // Copy msg.arrBodies array into the wasm instance, increase GC count
      const dataRef = wasm.__retain(wasm.__allocArray(wasm.FLOAT64ARRAY_ID, msg.arrBodies));
      // Do the calculations in this thread synchronously
      const resultRef = wasm.nBodyForces(dataRef);
      // Copy result array from the wasm instance to our javascript runtime